1. Field of the Invention
The present invention relates to an image scanning apparatus, an image forming apparatus, and an image scanning method, and, more particularly to an image scanning apparatus, an image forming apparatus, and an image scanning method that use a contact-type sensor.
2. Description of the Related Art
An image scanning apparatus such as a scanner and an image forming apparatus such as a copying machine include optical systems for scanning an original. The optical systems can be classified into a “reduction optical system” and a “contact optical system” (a CIS system: Contact Image Sensor).
The “reduction optical system” is a system for, while returning reflected light from an original many times with a mirror, leading the reflected light to a condenser lens and reading a reflection signal from the original with a sensor provided in a position apart from the original. Compared with the “contact optical system”, the “reduction optical system” has an advantage that a large depth of field can be secured. However, since an optical path length from the original to the sensor is large, in general, a physical length of the “reduction optical system” is large.
On the other hand, the “contact optical system” is a system for arranging a sensor under an original to be in contact with the original. The “contact optical system” has a disadvantage that a depth of field is small but has an advantage that a physical size thereof is small.
An original obtained by partially sticking two or more originals together (hereinafter referred to as stuck original) is included in originals to be scanned by the scanner or the copying machine. Since such a stuck original has a step in a stuck portion, the portion of the step may form a shade (hereinafter referred to as original shade) and appear as a black line in image data scanned.
In the case of the “reduction optical system”, since it is possible to irradiate direct light from a light source and reflected light from a mirror on the step of the stuck portion from different directions, an influence of the original shade is relatively small.
On the other hand, since the “contact system optical system” is a form in which a mirror is not provided, there is a problem in that, when an original is irradiated by one light source from a direction obliquely below the original, an influence of the original shade of the stuck portion is large.
In order to solve this problem, in the “contact system optical system”, a method of providing two light sources on the left and the right across a contact-type sensor and irradiating a step portion of an original from left and right two directions obliquely below the original to prevent occurrence of an original shade is considered.
However, since this method is a form in which two light sources are used, there is a problem in that power consumption increases.